Marine mammals and, particularly, cetaceans have long been a source of great interest. Their uncanny ability to navigate and feed under conditions of near zero visibility is due to the fact that they "see" acoustically. In order to determine what frequencies or groups of frequencies contribute to the animals' high resolution, the auditory thresholds at discrete frequencies within the spectrum of the projected and reflected signals should be known. This can be determined after an animal is safely secured and kept moist to avoid injury. Electroencephlograph (EEG) electrodes are properly placed and acoustic energy at discrete frequencies within an expected frequency range is projected toward the animal. Each time a frequency is projected and the animal hears it, a potential is involuntarily evoked. The potential is picked up by the EEG electrodes and fed to monitoring and recording circuitry. However, since the evoked potentials are minute in amplitude, in the range of less than 1.0 .times. 10.sup.-5 volts, they are difficult to detect. Merely, making the equipment more sensitive is not the answer since much greater potentials are picked up by the EEG electrodes whenever the animal moves. This movement occurs as the animal shifts about on its padded stretcher, or, as is much more common that the potentials attributed to movement, the movement artifact, are caused by the animal's opening and closing its blow hole. Whenever the movement artifact is present, the monitoring equipment responds to the generated potentials and the potentials attributed to the evoked response are obscured. It is obvious, therefore, that during the time when the animal is shifting or its blow hole is opening and closing, evoked response potentials to an acoustic stimulus are compromised. Even if several responses to the same frequency of acoustic stimuli were averaged without any movement artifact present, the artifact present during another response, when averaged with the others, would greatly distort or override the accumulated responses for a particular frequency of interest. Therefore, there is a continuing need in the state-of-the-art for an apparatus capable of measuring the auditory thresholds of marine mammals by recording the evoked potentials of auditory stimuli which is not compromised by the potentials atributed to the movement artifact.